1. Field of the Invention
The present invention relates to a display control apparatus for a display panel with one picture element constituted of four pixels of, for example, R (red), G (green), B (blue) and I (white), wherein the display control apparatus generates R, G, B and I signals from an input signal.
2. Related Background Art
Recently, display systems on the computer have been put to practical use, wherein display contents are represented not only by characters or lines but also natural images with characters and lines synthesized. This means that the feature of halftone display is important for the display panel (CRT, liquid crystal, plasma, EL) which is the display means in the display system. However, in display apparatuses except for CRT, the halftone display is not a simple matter. In the following, the halftone display on a liquid crystal display, particularly with a ferroelectric liquid crystal display panel will be exemplified.
Conventionally, display elements using a ferroelectric liquid crystal (FLC) are well known, wherein ferroelectric liquid crystal is injected into a liquid crystal cell having two sheets of glass substrate disposed opposedly with a cell gap of about 1 to 3 .mu.m held, its opposed faces being formed with transparent electrodes and subjected to orientation treatment, as disclosed in Japanese Patent Application Laid-Open No. 61-94023.
The features of such a display element using ferroelectric liquid crystal include the fact that a bonding strength between an external electric field and spontaneous polarization can be used for switching, and that the switching can be effected with the polarity of the external electric field, as the longitudinal directions of ferroelectric liquid crystal molecules corresponds one-to-one to the spontaneous polarization directions thereof. The ferroelectric liquid crystal is utilized mainly for the binary (white, black) display elements by making two stable states of light transparent and interrupting.
Further, typically, color display apparatus are well known wherein color filters of red (R), green (G) and blue (B) corresponding to the size of an electrode are provided on the glass substrate, one picture element being constituted of three pixels of R, G and B. Also, in order to improve the color characteristic of a panel, by providing two G pixels, one picture element may be constituted of four pixels of R, G, B and I. Further, in order to compensate for decreased brightness of the panel which may be caused by the low light transmittance of color filter and liquid crystal itself, a color display apparatus has been proposed in U.S. patent application Ser. No. 968,402 (filing date Oct. 29, 1992) wherein, by providing a white (I) pixel by means of a white color filter, one picture element is constituted of four pixels of R, G, B and I. This I pixel acts to increase the number of display colors.
FIG. 2 shows the relationship between the switching pulse amplitude of a ferroelectric liquid crystal element and the transmittance. This is a graphic representation in which the quantity of transmitted light after applying a single pulse with one polarity to a cell (element) in a complete light interrupted state (black) is plotted as the function of the amplitude V of a single pulse. When the pulse amplitude is equal to or less than a threshold V.sub.th (V&lt;V.sub.th), no quantity of transmitted light will arise, in which the transparent state of the pixel after applying the pulse as shown in FIG. 3B is not different from that of FIG. 3A indicating a state before applying the pulse. If the pulse amplitude V exceeds the threshold (V.sub.th &lt;V&lt;V.sub.sat), a part of the pixel transfers to the other stable state, resulting in a light transparent state as shown in FIG. 3C indicating a halftone of transmitted light as a whole. If the pulse amplitude V is further increased, exceeding a saturation value V.sub.sat (V.sub.sat &lt;V), the whole pixel is placed in a light transparent state as shown in FIG. 3D, with the quantity of transmitted light being fixed.
As can be seen from FIG. 2 to FIG. 3D, it is requisite that the pulse amplitude V be controlled to be V.sub.th &lt;V&lt;V.sub.sat in order to effect halftone display in the ferroelectric liquid crystal element. However, owing to a steep slope in a range from V.sub.th to V.sub.sat, it is difficult to control the halftone correctly with the pulse amplitude V.
This problem has been described in connection with FLC, but the same thing can be said if more halftone levels are to be obtained for a TN liquid crystal having no active elements.
In order to resolve the above problem, a method has been proposed in which pseudo-halftone display is enabled by using only two states as shown in FIGS. 3B and 3D. At present, most color display systems are constituted by CRT as the display apparatus, but when they are constituted by FLC which is difficult to make halftone display, instead of CRT, a pseudo-halftone display function, as previously mentioned, may be provided within the display apparatus for the purpose of providing the compatibility with the CRT and the general utilization as the display system, as described in U.S. patent application Ser. No. 968,402, cited previously.
However, the color characteristic of a panel as display means may greatly change by the area ratios of pixels such as R, G, B and I, the wavelength of backlight or its distribution, but as the display means itself outputs no information concerning the color characteristic to the outside, it was necessary to change the digital processing for pseudo-halftone display depending on the panel to prevent the color tint of an image from varying when the display means was changed.
Also, depending on the display panel such as FLC in particular, the display characteristic, particularly the color characteristic sometimes changed with the change in temperature caused by the use.
Further, the display characteristic changed with the filter arrangement or dot density of the display panel.